Jeremy Piven’s wild tale that mercury poisoning forced him to walk out on a popular Broadway show got even fishier yesterday.

The party-boy actor’s controversial doctor claims that his condition was caused by years of sushi consumption – and that his uncontrollable taste for tuna crippled him to the point he needed to be hooked off the stage of “Speed-the-Plow.”

“He’s been eating sushi twice a day for years,” said Dr. Car lon Colker of the Peak Well ness Center, who was sued in 2003 for allegedly making up data to mask the side effects of the dietary drug ephedra.

“He’s a voracious sushi eater. I tested him, and he’s got a shocking level of almost six times the upper limit of what’s allowable,” he said, referring to mercury in Piven’s system.

Colker said he “put the hammer down” and ordered Piven off the job.

“He had difficulty lifting his arms and legs off the bed . . . A few days later, he started experiencing dizziness and had trouble with his lines,” Colker said.

Piven ditched the show on Tuesday, claiming to be too ill to go on – angering skeptical castmates and investors banking on the “Entourage” star’s name to sell tickets.

“Skeptical? You could certainly say we’re skeptical,” said one investor of Piven’s excuse.

The producers are seeking a second opinion.

“We have recourse to have Jeremy examined by another doctor, and we are going to exercise that,” another investor said, hinting at legal action.

A third investor lamented, “We didn’t have star insurance, but we should have had a- -hole insurance.”

Piven’s “Speed-the-Plow” co-stars weren’t exactly behind him.

“We have not heard from him. We do not know where he is,” a visibly angered Raul Esparza said outside the Barrymore Theatre after an emergency rehearsal with Piven’s replacement. “It’s very disappointing when your co-star leaves.”

Asked about Piven’s claim of mercury poisoning, Esparza replied, “[If] that’s what he says, that’s what he says.”

Producers have tapped Oscar-nominated and Emmy-winning actor William H. Macy to take over Piven’s role in two weeks. For now, Tony-winner Norbert Leo Butz is the emergency fill-in.

Producers were first alerted to Piven’s desire to leave a few weeks ago, after he called his friend, actor Steven Webber, whom he asked to replace him.

Piven also personally called prominent casting directors to see whether they could line up other actors.

“I have never heard of a star personally looking for his replacement,” said an investor.

Piven told friends who visited him backstage he was “bored out of his mind” and couldn’t wait to get out, sources say.

Producers had already been suspicious about Piven’s after-show whereabouts – he’d often been spotted at various bars – asking the car service hired to take him home to report back. A peeved Piven had an assistant call to demand a new car service, sources said.

A week ago, he started showing up minutes before the curtain went up rather than the mandatory 30 minutes.

Onstage, he’d ad-lib whenever latecomers arrived, much to the dismay of co-stars Esparza and Elisabeth Moss.

Over the weekend, producers were informed Piven had collapsed in his apartment and had been hospitalized. While they tried to find out where, it turned out he was actually at Colker’s wellness center instead.

Colker has been the target of lawsuits in Missouri, West Virginia and Illinois over the dietary supplement ephedra. His lawyer said he was dropped from all three actions before trial.